1 Answer
1

It is Shift+3 within a message for Mac.
It is the same in the inbox as well

You must move to the message with j/k

Select a message with x

Then press Shift+3

It could be that your keyboard layout is different.
Using a UK layout, it would be Option + 3 : ⌥ + 3, but this does not work (Not sure why).
You can also remap your keys with the Google Labs feature : Custom keyboard shortcuts